The ghosts that haunt Sen. John McCain are about 600 in number and right now, they are mustering for a final onslaught. McCain, one of America’s foremost Republicans and President Barack Obama’s opponent in 2008, is currently locked in a desperate bid for political survival in his home state of Arizona.

After 20 years of immunity from challenge from his fellow Republicans, he’s now involved in a close primary battle with J.D. Hayworth, a former congressman turned radio broadcaster who sports the tea party label. Hayworth says McCain is a fake Republican, soft on issues like immigration. The polls have been tightening, and if McCain got bludgeoned by some new disclosure, it could finish him off.

That very disclosure is now likely to burst over the head of McCain, the former Navy pilot who was held in a North Vietnamese prison for five years and returned to the U.S. as a war hero.

His nemesis is Sydney Schanberg, a former New York Times reporter who won a Pulitzer Prize for his reporting from Cambodia that formed the basis for the Oscar-winning movie “The Killing Fields.”

In recent years, Schanberg has worked relentlessly on one of the great mysteries of the Vietnam War, one that still causes hundreds of American families enduring pain. Did the U.S. government abandon American POWs in Vietnam?

By 1990, there were so many stories, sightings and intelligence reports of American POWs left behind in Vietnam after the war was over that pressure from Vietnam vets and the families of the MIAs (missing in action) prompted the formation of a special committee of the U.S. Senate to investigate. The chairman was John Kerry, a Navy man who had served in Vietnam. McCain, as a former POW, was its most pivotal member.

Down the years, Schanberg has pieced together the evidence, much of it covered up by the Senate committee. In 1993, an American historian unearthed in Soviet archives the record of a briefing of a Vietnamese general to the Soviet politbureau. The briefing took place in 1973, right before the final peace agreement between the U.S. and Hanoi.

What the Vietnamese general told the Russians was that his government was intent on getting war reparations, $3.25 billion in reconstruction money, pledged by the U.S. in peace negotiations headed on the U.S. side by Henry Kissinger. The general told the Russians that Hanoi would hold back a large number of POWs until the money arrived.

But Nixon and Kissinger had attached to the deal a codicil to the effect that the U.S. Congress would have to approve the reparations — which the two knew was an impossibility in the political atmosphere of the time. Thus they effectively sealed the POWs’ fate. Hanoi released 591 immediately, but held back around 600.

All of this was suppressed by the Kerry-McCain committee, with the complicity of the U.S. press, enamored of both McCain and Kerry. McCain was particularly vicious in mocking what he and his press allies suggested were the fantasies of MIA families and Vietnam vets.

Schanberg writes now that, “In a private briefing in 1992, high-level CIA officials told me that as the years passed and the ransom never came, it became more and more difficult for either government to admit that it knew from the start about the unacknowledged prisoners. Those prisoners had not only become useless as bargaining chips but also posed a risk to Hanoi’s desire to be accepted into the international community.

“The CIA officials said their intelligence indicated strongly that the remaining men — those who had not died from illness or hard labor or torture — were eventually executed.”

In the presidential campaign of 2008, as I reported for The First Post at the time, McCain faced accusations that in fact, as a POW, he had broken and cooperated with his North Vietnamese captors, who regarded McCain as a valuable prize because his father was a prominent U.S. admiral, at the time commander of all U.S. forces in the Pacific.

McCain Jr., so his accusers said, disclosed vital information and made broadcasts denouncing the U.S., which were then used by the Vietnamese to break other POWs.

The issue never became a big one in 2008 — but now it’s coming on back with a vengeance.

On May 26, the American Conservative, a monthly, released a special issue, “The Men our Media Forgot.” The U.S. media, pressured in any number of ways by successive U.S. governments to ridicule and suppress enquiries into the missing POWs, are the prime target, but McCain also bulks large in the American Conservative’s sights, since his present political crisis forms an excellent peg for Schanberg’s story. The calculation is evidently that this could be a huge boost to Hayworth.

In an article for the American Conservative titled “McCain and the POW Cover-Up,” Schanberg insinuates, without saying so directly, that the Pentagon blackmailed McCain to squelch the MIA hearings: “It’s not clear whether the taped confession McCain gave to his captors to avoid further torture has played a role in his postwar behavior in the Senate. That confession was played endlessly over the prison loudspeaker system at Hoa Lo — to try to break down other prisoners — and was broadcast over Hanoi’s state radio.

“Reportedly, he confessed to being a war criminal who had bombed civilian targets. The Pentagon has a copy of the confession but will not release it. Also, no outsider I know of has ever seen a non-redacted copy of the debriefing of McCain when he returned from captivity, which is classified but could be made public by McCain.”

Alexander Cockburn is co-editor with Jeffrey St. Clair of the muckraking newsletter CounterPunch. He is also co-author of the new book “Dime’s Worth of Difference: Beyond the Lesser of Two Evils,” available through http://www.counterpunch.com.